Multilayer laminates formed by laminating a polyamide and a fluorine-containing resin are expected as composite materials having polyamide characteristics, such as high strength, high toughness, light weight, good processability and in particular, flexibility, as well as fluorine-containing resin characteristics, such as heat resistance, oil and chemical resistance, and low liquid agent permeability.
The successive lamination method and simultaneous lamination method are known for producing such multilayer laminates. Among them, the simultaneous lamination method, especially coextrusion method holds an important position because of its merit lying in that the number of steps required for obtaining the laminate can be lowered. The simultaneous multilayer coextrusion method is applied to multilayer molding or multilayer film formation using two or more extruders and, currently, films, sheets, extruded molded articles called profiles, pipes, hoses, tubes and other moldings varied in shape or form are being produced by the method. The method is applicable to various thermoplastic resins, such as polyamides, as well as to fluorine-containing resins.
However, fluorine-containing resins are characterized in that they are low in intermolecular cohesive force and very low in surface free energy because of the low polarizability of fluorine-containing molecules. Therefore, they can hardly be wet against solids higher in intermolecular cohesive force, hence they show low adhesive properties against most of other substances. They, as resins, are thus characteristically poor in adhesion properties and their interlayer adhesiveness with polyamides is low; it is therefore necessary to contrive to increase the interlayer adhesion strength. For example, methods are known which comprise subjecting the surface of the fluorine-containing resin to surface treatment such as corona discharge treatment or irradiation treatment. Japanese Kokai Publication Hei-05-8353, for instance, proposes a method of producing multilayer tubes comprising a polyamide resin as an outer layer and a fluorine-containing resin as an inner layer by irradiating the tubes to introduce intermolecular crosslinking structures among molecules in both layers and thereby secure the adhesion strength between the polyamide resin layer and the fluorine-containing resin layer. However, the method cannot be applied to the simultaneous lamination technique including coextrusion.
Also available is a technology according to which a polymer blend comprising a fluorine-containing resin is used for the resin layer to be adhered to a polyamide. As a production method using this technology, Japanese Kokai Publication Hei-07-53823 discloses a method of producing multilayer tubes comprising an outer layer comprising a polyamide and an inner layer comprising a fluorine-containing resin in which a resin composition comprising both a specific polyamide and a fluorine-containing resin is laminated to a polyamide layer so that it may serve as an adhesive layer with the inner layer. In this production method, however, the morphology of the adhesive layer changes according to the molding conditions due to the intrinsically poor compatibility between the constituent resins of the adhesive layer, namely the polyamide and fluorine-containing resin, and the morphology change influences on the cohesive force of the adhesive layer itself and the adhesion strength thereof with the other layers. Thus, such problems arise as a tendency toward variation in adhesion strength according to environmental factors such as molding conditions and temperature conditions during use and difficulty in securing a constant quality level. Furthermore, this technology does not intend to increase the adhesiveness of the polyamide and fluorine-containing resin themselves but merely utilizing the adhesiveness of the polymer blend. In addition, the use of such polymer blend in lieu of the fluorine-containing resin results in an impairment in those excellent characteristics of the fluorine-containing resin.
To solve this problem, attempts have been made to improve the fluorine-containing resin itself, and various fluorine-containing resin materials have been proposed. For example, in the WO 99/45044 pamphlet, there is disclosed a method of multilayer simultaneous extrusion of laminates which comprises polyamide 12 (nylon 12) as an outer layer and a fluorine-containing ethylenic polymer having carbonyl groups, such as carbonate groups and/or carbonyl halide groups, as an inner layer fluorine-containing resin to be laminated to the outer layer. In this method, the die temperature is set at 260° C., and an interlayer adhesion strength of ten and several of N/cm and a good appearance can be obtained. However, such a level of adhesion strength is not fully sufficient in some fields of application.
Thus, in the art, the efforts to develop production technologies of improving the performance characteristics of multilayer laminates comprising a polyamide and a fluorine-containing resin, in particular a fluorine-containing ethylenic polymer, can be said to have been principally directed to two aspects, namely investigations concerning the method of physical treatment of the adhesive surface and contrivances for adhesive materials, as mentioned above. There is no production method known, however, for attaining sufficiently high interlayer adhesion strength without requiring an additional step or steps and without being restricted to a specific adhesive material, in particular for attaining high productivity by applying the simultaneous multilayer coextrusion technique.